Olefinic lower hydrocarbons such as propene, butenes and isobutene are very important intermediates in the petrochemical industry. Such olefins are primarily produced as co-products in catalytic and steam cracking processes. Alternatively, lower olefins can be commercially produced by catalytic dehydrogenation of the corresponding lower alkanes. U.S. Pat. No. 3,763,255 for instance describes a method for dehydrogenation of C4-C30 hydrocarbons using a catalyst which comprises a platinum component, an iridium component and an alkali or alkaline earth metal component with a porous carrier material. The applicability of conventional endothermic dehydrogenation of lower alkanes, however, is limited by thermodynamic constraints and rapid catalyst deactivation caused by coke formation.
Oxidative dehydrogenation (ODH) of lower alkanes is a potentially advantageous alternative due to the exothermic nature of the overall reaction. Different supported and unsupported catalyst compositions useful in the oxidative dehydrogenation of C3-C5 hydrocarbons in general and of isobutane in particular have been described. A critical aspect of such catalyst compositions is the selectivity for the produced olefin (e.g. isobutene), since the formation of carbon oxides is thermodynamically much more favourable than the formation of olefin.
U.S. 2004/0010174 A describes a process for ODH of hydrocarbon feedstocks having from 2 to 10 carbon atoms characterized in that a circulating fluidized bed/regenerator system is employed. Accordingly, a mixed metal oxide catalyst is reduced in the ODH reactor and the reduced catalyst is transported to a separate regenerator system for reoxidation with air. The catalyst employed in the process according to U.S. 2004/0010174 A has the general formula αAOx-βBOy-γCOz, wherein A is a precious metal and/or transition metal; B is a rare earth metal; C is an element chosen from Groups IIA, IIIA and IVA; and O is oxygen. α, β and γ are the relative molar ratios of each metal oxide. U.S. 2004/0010174 A remains silent with respect to the selectivity and the yield of the disclosed process.
In EP 1166869 A a method for ODH of hydrocarbons is described comprising contacting a hydrocarbon with a tungsten-based catalyst composition in the presence of molecular oxygen at 350° C. to 550° C. The tungsten-based catalyst composition of EP 1166869 A has the general formula XxYyWOz, wherein X is at least one element selected from the group consisting of Li, Na, K, Rb, Cs and Fr; Y is at least one element selected from the group consisting of B, Al, Ga, In, Ti, C, Si, Ge, Sn and Pb; W is tungsten; and O is oxygen and wherein x is 0.5-2.5, y is 0.05-5, and z is the number to satisfy the valency of X, Y and W. When using the catalyst composition Na2SiWOx as catalyst at a reaction temperature of 500° C., an isobutene selectivity of 70.0 and an isobutene yield of 8.2 could be achieved.
Supported chromium-based catalyst compositions that are commercially used in endothermic catalytic dehydrogenation of lower alkanes may also be used in oxidative dehydrogenation (ODH) processes.
In GRABOWSKI, et al., Oxidative dehydrogenation of isobutane on supported chromia catalysts, Applied Catalysis. 1996, vol. 144, p. 335-341, for instance, the use of chromia supported on silica (CrOx/SiO2), alumina (CrOx/Al2O3), titania (CrOx/TiO2), zirconia (CrOx/ZrO2) and magnesia (CrOx/MgO) as a catalyst for ODH of isobutane in the presence of molecular oxygen is described. The highest isobutene selectivity (70% at 5% conversion) and isobutene yield (9%) could be obtained by using CrOx/TiO2 and K-promoted CrOx/Al2O3 catalyst compositions.
SUN, et al., Kinetics of the oxidative dehydrogenation of isobutane over Cr2O3/La2(Co3)3, J Nat Gas Chem. 2002, vol. 11, p. 70-78 describes the use of chromia supported on lanthanum carbonate (Cr2O3/La2(CO3)3) as a catalyst for ODH of isobutane in the presence of molecular oxygen. By using said Cr2O3/La2(CO3)3 catalyst, an isobutene selectivity of 95% and 12.5% conversion at a reaction temperature of 230-250° C. can be achieved.
Besides by selecting the catalyst composition, the selectivity and/or yield of a process for ODH of a lower alkane can be influenced by the composition of the feedstream. As described in SUN, et al, Kinetics of the oxidative dehydrogenation of isobutane over Cr2O3/La2(Co3)3, J Nat Gas Chem. 2002, vol. 11, p. 70-78 for instance, ODH of isobutane to isobutene over Cr—Mn—O/Al2O3 is three times greater in the presence of carbon dioxide in the reaction mixture. More particularly, KRYLOV, et al, Heterogeneous catalytic reaction of carbon dioxide, Russian Chemical Reviews 1995, vol. 64, p. 877-900 describes that in the presence of a Cr—Mn—O/Al2O3 catalyst isobutane conversion reaches 61-66% at an isobutene selectivity of 78-81%. Equal amounts of carbon monoxide and hydrogen are further produced in said ODH reaction.
In RU 2035444 C a process for conversion of isobutane to isobutene in the presence of carbon dioxide at 610-680° C. is disclosed, wherein the molar ratio of isobutane to carbon dioxide in the feed is 0.8-1.4:1. The employed catalyst contains 2-6 mass % Cr and 2-6 mass % Mn supported on γ-Al2O3.
Also in U.S. 2004/0181107 a process for alkane dehydrogenation is disclosed which comprises contacting an alkane with a chromium-based dehydrogenation catalyst in the presence of carbon dioxide. The molar ratio of alkane and carbon dioxide is about 1:0.0001 to 1:0.045.
Although reasonable isobutene yields can be obtained by employing chromium-based dehydrogenation catalysts, catalyst performance is not stable due to coke formation and decreases within several hours. As a further disadvantage, chromium-based catalysts are environmentally problematic due to the high toxicity of many chromium compounds.
The technical problem underlying the present invention is the provision of a process for oxidative dehydrogenation (ODH) of lower alkanes having a high selectivity and yield of the produced olefin and wherein the used catalyst has an improved stability against deactivation and is less environmentally problematic when compared to chromium-based catalysts.